In recent research carried out in recanalization of atherosclerotic vessels, the laser heated cautery cap has been proven as an effective tool in reopening stenotic vessels. Moreover, it has been demonstrated that atherosclerotic plaque can be removed by attacking it with a laser heated cautery cap that is heated briefly and reiteratively to a high temperature sufficient to thermally destroy the plaque material. In this approach the brief thermal pulse delivered to the plaque destroys the plaque in contact with the cap before the heat can flow to the arterial walls, so that damage to the vessel is minimized or eliminated.
The brief, high temperature thermal pulse can be delivered only by a laser heated cautery cap having a small thermal mass, so that heating and cooling of the cap is accelerated to the maximum extent. However, reduction of the thermal mass in a conventional cautery cap requires that less physical mass is present, and there is a limit to the amount of cap mass that can be eliminated before structural problems occur. For example, the cap wall thickness cannot be reduced below the minimum required to prevent collapse of the cap. Moreover, reduction in the cap mass generally requires smaller cap parts, which exacerbates assembly and manufacturing problems.
It has also been found that the best results are achieved in recanalization by laser heating cautery cap if the high temperature thermal pulse can be localized in the cap areas that are in contact with the plaque, while preventing thermal flow to the remainder of the cap. This requirement has been met in the prior art by multiple fiber catheter assemblies, with each fiber heating a discrete portion of the cap. However, directing the laser energy solely to the leading shoulder area of the cap which generally contacts the plaque, while avoiding unacceptable heating of the vessel wall, is a goal that is not always attained in prior art cautery cap structures.
The related patent application noted above describes one solution to the problems discussed above, in that it describes a laser heated cautery cap formed by a transparent substrate and outer layer applied directly to the substrate surface by deposition techniques. The transparent substrate is a crystalline solid, such as sapphire, that is highly transparent to the optimum laser type for this field (NdYAG), rugged and dimensionally stable over a high temperature range, easily machined and surfaced, and is a thermal insulator. Portions of the substrate are coated to absorb the laser light received from a plurality of optical fibers, while the remaining portions are coated with a highly reflective coating to keep all laser light captive and to minimize surface heating of these remaining portions. The present invention includes improvements in these concepts, as well as new structural and functional additions.